The present invention relates to a rodless cylinder, that is, a cylinder with no piston rod, and more particularly to a rodless cylinder which is capable of minimizing the leak of pressure fluid.
Conventionally, there has been known a rodless cylinder of the type which is constructed such that a slit of a trapezoidal section is formed in a cylinder tube in a longitudinal direction thereof, a trapezoidal belt which is fixed to the ends of the cylinder tube at the both ends thereof is fitted into the slit, and the trapezoidal belt is brought into engagement with a piston which is slidably inserted and fitted into the cylinder tube so as to seal the slit in the cylinder tube with the trapezoidal belt.
However, the conventional rodless cylinder of the above mentioned type has such a drawback that an area between the piston and the inner peripheral surface of the cylinder tube is sealed with a piston cup (a ring-type lip packing) which is fitted onto the outer periphery of the piston, and a part of the piston cup which is in press contact with the horizontal plane of the bottom of the trapezoidal belt deforms with a curvature which is different from that of the inner peripheral surface of the cylinder tube, or when the trapezoidal belt is fitted into the slit, the trapezoidal belt does not align with the bottom of the slit due to the tolerance in size and hence a level difference is induced between the bottom of the trapezoidal belt and the inner peripheral surface of the cylinder tube, so that the pressure fluid which is applied into the cylinder tube leaks out through the gap created by the level difference.
Accordingly, the inventors of the present invention have proposed a rodless cylinder of the structure as shown in FIG. 9 in Japanese Laid Open Patent Specification No. 184208/87. This rodless cylinder is constructed such that a rib 21c is provided between outer lip 21a and inner lip 21b of a piston cup 21 which is fitted onto the outer periphery of a piston 20 in such a manner that the outer lip 21a is pushed against the inner peripheral surface of a cylinder tube 22 by the rib 21c. Further, the rodless cylinder is constructed such that concavely curved surfaces are formed on the both sides of a trapezoidal belt 23 in such a manner that when the base of the trapezoidal belt 23 is pushed by the piston cup 21, the base can be readily deformed after the curve of the inner peripheral surface of the cylinder to prevent the outer lip 21a from being inclined, thereby to reduce the leak of the pressure fluid.
However, the above mentioned conventional rodless cylinder has such a drawback that as shown in FIG. 10. an angulated portion A is formed in a position of the trapezoidal belt 23 entering a slit 24 from the inner peripheral surface of the cylinder tube 22, so that a lower angulated portion of the trapezoidal belt 23 slightly projects inwardly from the inner peripheral surface of the cylinder tube to induce a level difference due to the tolerance in size of the trapezoidal belt 23 and the slit 24, a gap C is created between the outer lip 21a of the piston cup 21 and the inner peripheral surface of the cylinder tube and hence the pressure fluid is liable to leak through the gap C.
Therefore, there has been demanded a rodless cylinder which is constructed so as to further reduce the leak of the pressure fluid out of the cylinder tube 22.